Increases in the strength of cannabis are mainly due to new production techniques in Morocco and Europe.
Photograph: Don Mackinnon/AFP/Getty

Cannabis potency has doubled across Europe in the past decade, according to the first study to track changes in the drug across the continent.

The study, published on Sunday in the journal Addiction and conducted by researchers from the University of Bath and King’s College London, finds that both cannabis resin and herbal cannabis have increased in strength and price with potentially harmful consequences for users.

In herbal cannabis, concentrations of THC – the main psychoactive constituent of cannabis which has been linked to psychosis – increased from 5% in 2006 to 10% in 2016.

For cannabis resin, THC concentrations remained relatively stable between 2006 and 2011 before increasing rapidly from 10% to 17% between 2011 and 2016.

The price of herbal cannabis increased from €7.36 per gram to €12.22 between 2006 and 2016 while the price of cannabis resin increased from €8.21 per gram to €12.27 per gram over the same period.

In the UK alone, THC levels in herbal cannabis remained roughly similar between 2006 and 2016, however police seizures suggest they have risen sharply in cannabis resin.

“These findings show that cannabis resin has changed rapidly across Europe, resulting in a more potent and better value product,” said Dr Tom Freeman from the Addiction and Mental Health Group within the Department of Psychology at the University of Bath who led the study, which was funded by the Society for the Study of Addiction.

Increases in the potency of cannabis resin are chiefly down to new production techniques in Morocco and Europe. But while THC levels have increased, amounts of cannabidiol (CBD) found in cannabis resin thought to offset some of the harmful effects of THC – have remained stable or declined.

“CBD has the potential to make cannabis safer, without limiting the positive effects users seek,” Freeman said. “What we are seeing in Europe is an increase in THC and either stable or decreasing levels of CBD, potentially making cannabis more harmful.”